In modern wireless transmission systems such as, for example, DECT, GSM, EDGE, IS95, WCDMA 2000, integrated circuits are required which reliably convert a received analog signal into a digital signal which can be processed further.
FIG. 1 diagrammatically shows the components of a conventional receiver arrangement. An antenna AT receives an emitted signal and delivers an antenna signal A1 to a preamplifier LNA (low noise amplifier) which provides a preamplified signal A2. An input filter IF usually arranged as band-pass filter, filters the preamplified antenna signal A2 and delivers a filtered received analog signal A3 to a mixer device M1, M2 which in each case mixes the filtered received signal A3 with the local-oscillator frequency LO and outputs a respective mixer current signal A4, A4′. FIG. 1 shows an upper mixer branch M1 and a lower mixer branch M2, the mixed signals in each case being phase shifted by 90°.
The mixer is followed by an adjacent-channel filter NF, NF′ which is usually arranged as a low-pass filter, for example as a fourth- or fifth-order Butterworth filter. The adjacent-channel filter essentially prevents the received signal A4 mixed with the local-oscillator frequency LO from being interfered with by adjacent channels or frequencies.
The received signal A5, A5′ filtered in this manner is in each case converted by an analog/digital converter ADC, ADC′ into a digital signal D, D′ which can be processed further in a baseband processor BBP and which reconstructs the corresponding transmitted digital data. When individual circuits are used for mixers, adjacent-channel filters and analog/digital converters, a number of disadvantages occur.
A conventional mixer has, for example, an amplifier, a switch device and a current/voltage converter. In this arrangement, the radio-frequency signal to be mixed is amplified and mixed by a switch device switched by the respective local-oscillator frequency. Following this, a current/voltage conversion is required in order to generate a mixed output voltage signal.
In this process, a normally used current/voltage conversion is effected via an operational amplifier which, at the same time, performs signal filtering by means of a feedback resistor and a parallel-connected capacitor. The corresponding feedback capacitors suppress a transmission of the radio-frequency input signal into the output signal of the respective operational amplifier. However, the feedback resistance value defines the gain of the mixer and the gain must only be adjusted moderately since otherwise the mixer output signal would be limited. However, a small resistance value increases the corresponding filtering bandwidth, since in an operational amplifier with capacitive and resistive feedback, the corresponding bandwidth is fbw α ½n·C·R, where R and C are the values of the corresponding feedback resistors and capacitors. A small resistance thus increases the correspondingly needed feedback capacitance and thus the area required by such a circuit on a semiconductor chip. For GSM applications, for example, the filter bandwidth should be about 100 kHz. With feedback resistances R≈1 kOhm, a large capacitance of C=100 pF−10 nF is thus necessary which requires a large area on the corresponding semiconductor chip and increases the current consumption of the mixer device.
Before the correspondingly mixed analog signal is conducted to an analog/digital converter, adjacent-channel signals must be reliably suppressed. To combine corresponding filtering with the analog/digital conversion, it has been proposed in the past to use time-continuous sigma/delta (analog/digital) converters.
In the sigma/delta conversion, a feedback signal is subtracted from an analog input signal to be converted and the resultant signal is first subjected to filtering, in most cases to an integration and is then quantized. The quantized digital output signal obtained in this manner is converted by a feedback-type digital/analog converter and used as feedback signal. The digital output signal of the quantizer, for example, mainly supplies a High level with a rising analog input signal, a Low level with a falling analog input signal and alternating High and Low levels with an essentially constant analog input signal. Simple digital integrating then supplies the digital signal value from the output signal.
One advantage of time-continuous filtering or integrating in the sigma/delta converter consists, in particular, in that an inherent filter function of the corresponding analog/digital converter exists. Between the input of a continuous sigma/delta converter and the quantizing input of the sigma/delta converter, an anti-aliasing filter frequently of a higher order is implemented, therefore, which can be used as adjacent-channel filter when the sigma/delta converter is used in a receiver circuit.
In chapter 4.4. in L. Breems, J. H. Huijsing: “Continuous-Time Sigma-Delta Modulation for A/D Conversion in Radio Receivers”, Kluwer Academic Publishers, 2001, ISBN 0792374924, for example, the combination of a mixer with a sigma/delta converter is presented.
FIG. 2 shows a corresponding diagrammatic representation of such a receiver arrangement. At the input end, a mixer M is provided which is supplied with the received analog signal A3 and which carries out mixing with the local-oscillator frequency LO. At the output of the mixer, a capacitor C1 is provided which ensures that the received radio-frequency signal A3 is decoupled. The value of this capacitor C1 must be selected to be high in order to achieve good decoupling. From the output current signal A4 of the mixer M, the feedback signal FB is subtracted which is supplied by a feedback digital/analog converter DAC. The input stage of a voltage-amplifying operational amplifier EV and a serial feedback loop of a resistor R1 and a capacitor C2 acts as integrator. The corresponding analog integrated signal A5 is supplied to a serial chain of transconductance integrators TV1, TV2, TV3, TV4. Between the transconductance integrators TV1, TV2, TV3, TV4, corresponding intermediate signals A5, A6, A7, A8 are picked up and weighted by transconductance amplifiers V1, V2, V3 in forward coupling loops FF1, FF2, FF3. The corresponding forward coupling signals F1, F2, F3 are combined with the signal A9 passing through the integrator chain TV1, TV2, TV3, TV4 at a node K located before the quantizer Q. The quantizer Q finally supplies the digital output signal D which is also supplied to the feedback digital/analog converter.
In the receiver circuit shown in FIG. 2, a signal path without further filtering is always obtained between the first integrator EV, R1, C2 and the input of the quantizer Q. This is the first feed forward branch FF1. Since the mixer M preceding the sigma/delta converter arrangement is present at the input of the integrator EV, R1, C2 and the corresponding signal A4 on the signal path is only subjected to first-order filtering, the input of the sigma/delta arrangement is sensitive to interference signals from the received signal A3.